Variable resistors are based on an electro-resistive element called Potentiometer track. A potentiometer is constructed using a resistive element, with a sliding contact (“wiper”), whereas the wiper can slide over the resistive element (“track”, “resistive path”). The end points of the resistive path provide two terminal end points and the wiper provides the third terminal.
Variable resistors are commonly used in an electronic circuit either in a 3 lead wires potentiometer mode or in a 2 wires rheostat mode. In the first mode, herein referred to as “potentiometer” mode, an input voltage is applied to end points of the track and the output is a partial voltage at wiper terminal, wherein the output voltage is a function of the position of the wiper on the track.
In the second mode, herein referred to as “rheostat” mode, a variable resistor is obtained between one end point terminal of the track and the wiper terminal, while the second end point of the track can be left open or short-circuited with the terminal post of the wiper.
A rheostat mode application is described in U.S. Pat. No. 5,343,188 ('188) given to Yasuda et al. Reference is now made to FIG. 1 (prior art), which is an electrical schematic of a slide rheostat 20 as provided by '188. Three resistors are produced on a common substrate 28 using the same resistive material: a resistor track having a total resistance of Rt, having two end point terminal 24 and two load resistors RL1 and RL2 (called “clamping resistors”) connected at each end point 24 of resistor Rt. Wiper 22 slides over track Rt, thereby dividing track Rt into two segments, whereas the resistance of the segment between the terminal of wiper 22 (“rheostat track”) and end point terminal 24a is Rx. Five possible wires can come out of slide rheostat 20: L1, out of RL1; L2, out of end point terminal 24a; LW, out of the terminal of wiper 22; L3, out of end point terminal 24b; and L4, out of RL2. The wires (L1, L2, L3. L4 and LW) are leading to a voltage source. A processing unit provides output voltages related to the position of wiper 22 and indicates that there is no short circuit or no open circuit in the rheostat track. The load resistors and the rheostat track have similar temperature dependence (TCR), but the difference between the temperatures depends on the wiper's position due to changing heat dissipation pattern.
The present invention mentions also the rheostat mode, but both the goal and the construction are different: two wires leading from Rx on the rheostat track to a remote load resistor allow a reduction in the weight of wiring and all resistors are of a high precision and stability class required to maintain the high accuracy of the electric output versus wiper's position relationship. The accuracy of the relationship depends on accuracy and stability of all components.
The requirements in applications of high precision variable resistors are:                a) High reliability over a long service life;        b) High precision of resistance value as measured between end points 24;        c) High stability of resistance when ambient temperature changes and after a long service life;        d) High resolution—capability of fine adjustment;        e) Linearity—how well the measured output fits the specified linear or other function of electrical output versus the position of wiper 22; and        f) Low and stable contact resistance of wiper 22.        
Four prevalent technologies used in production of the tracks are cermet (paste of glass and conductive particles, screen printed and fired), composition (polymer with conducting particles), wire-wound and foil. In prior art thin film technology is not used because the thin film is easily abraded by the moving wiper 22.
The first two are not suitable for high stability applications due to two types of changes in the resistance values: a reversible change with changing temperature (high temperature coefficient of resistance (TCR)) and a permanent one, with load and time. Cermet resistors have better specifications than composition and have TCR between 50 and 250 ppm/° C. and tolerances of 1% to 5%.
For fixed value resistors of higher precision, thin film technology provides TCR between 5 and 25 ppm/° C., but is not used in variable resistors because the thin film is easily abraded by the moving wiper.
The fixed value foil resistors are available with TCR down to less than 1 ppm/° C. and tolerances down to 0.001%, and foil variable resistor tracks (see ref. 2) provide a high stability and resolution, but have a limited service life and are used only as precise trimming potentiometers (trimmers) which are only occasionally adjusted.
According to prior art, some increase in life expectancy is achieved by use of special lubricants.
Precision foil resistor with very low TCR are described in U.S. Pat. No. 4,677,413 ('413), given to Felix Zandman et al, the disclosure of which is incorporated herein by reference for all purposes as if entirely set forth herein.
U.S. Pat. No. 3,821,845 ('845), Given to Hukee et al, describes a method by which a computerized probing board is mapping sections of the resistive film and a laser beam is cutting into the film to increase the segment's resistance. Such cutting mode introduces instability of the resistance value by creating heat affected zones at the end of laser cuts, were the current density is increased. The resistance drift, obtained after a laser cut is also described in IEEE Transactions on Components, Hybrids, & Manufacturing Technology by A. Kestenbaum et. Al., December 1980, V.3, Is. 4, pages 637-648, “Trimming Behavior and Post-Trim Characteristics of Ta2N Resistors on Silicon”, which is incorporated herein by reference.
Typically, wire-wound tracks have poor resolution: the device lacks electrical and mechanical smoothness of operation—wiper 22 jumps from one wire to the next, creates an electrical output of large steps as a function of the position of wiper 22 and fine adjustment is not possible. Wire-wound tracks are mainly used for higher power applications
U.S. Pat. No. 3,601,744 ('744), given to Felix Zandman, and U.S. Pat. No. 3,405,381, given to Felix Zandman et al, the disclosure of which are incorporated herein by reference for all purposes as if entirely set forth herein, suggest resistors having foil tracks that provide a high stability and resolution, but have a limited service life and are used only as precise trimming potentiometers (trimmers) which are only occasionally adjusted.
There is therefore a need and it would be advantageous to have a high accuracy variable resistor and a two-wire position sensor, the latter especially for applications of position sensing requiring reduction of the weight of wiring. Furthermore, it would be advantageous to provide variable resistor tracks of long service life, high precision, high stability, low contact resistance, high resolution and linearity using existing precision resistors production technologies.
The quality of variable resistors in terms of precision, stability and length of service life is today limited due to characteristics of resistive materials employed and their abrasion by a wiper moving on the resistive path. Attempts are made to remove the debris created by this abrasion but it doesn't reduce the resistance shift caused by the abrasion. U.S. Pat. No. 5,258,737, given to Roland Wanja, provides a potentiometer, which avoids problems caused when abraded material which collects on the surface of the potentiometer track as a result of friction between the wiper contact and the potentiometer track, causing an undesirable increase in the contact resistance. In another example international patent application WO 1955/009428 by Michael Cairns et al, provides a potentiometer less liable to produce transient voltage spikes caused by its contacts being lifted from the track of electro-resistive material when encountering detritus or dust uses slider contacts having divergent contact areas.
Prior art potentiometers for long term life use resistive materials of type not suitable for high precision and stability, which characteristics can be met by the use of thin film and even more so by foil technology, but these are not suitable for long term abrasion by a sliding wiper. FIG. 2 (prior art) illustrates potentiometer track 30, having two terminal pads 34 and a meandering resistive path 36 composed of vertical lines which are connected by top end loops 38a and bottom end loops 38b. 
In a rotary potentiometer a flexible substrate enables bonding of such a track to the wall inside a cylindrical housing, and a contacting wiper attached to a rotating arm slides along track 30 and contacts also an electrical output terminal on the housing. Alternative designs use a planar track with an arc shaped pattern (see '744).
FIG. 3 (prior art) illustrates potentiometer track 40, having two terminal pads 44 and two meandering resistive paths 46, respectively, connected in parallel by end-loops 48: Bottom end-loops 48b of the upper meander path 46 contact top end-loops 48a of the lower meander path 46b, at the horizontal center-line of potentiometer 40. Such arrangement of two (or more) meanders 46 in parallel has the advantage of avoiding a catastrophic failure caused by a discontinuity in the resistive path. To cause an open circuit, both meanders 46 must be cut between two adjacent end loops common to the two meanders. Addition of parallel meanders causes also a reduction of the track's resistance value.
FIG. 4 (prior art) illustrates potentiometer track 50, having two terminal pads 54, meandering resistive path 56 and a collector bar 52 having a lead wire connected to collector bar 52 and serving as the output terminal. Collector bar 52, having an output lead wire, is serving as the third terminal. The resistance of collector bar 52 plays a minor role in the potentiometer mode, but may be harmful in the rheostat mode as it adds to wiper's 22 contact resistance.
There is therefore a need and it would be advantageous to be able to eliminate the wear of the resistive material and to enable the use of high precision and stability resistive materials.